This invention relates to the use of cryocoolers in superconducting magnets for magnetic resonance imaging (hereinafter "MRI"), and more particularly to noise reduction, and vibration isolation, of the cryocooler to improve the quality of the images provided by MRI operation.
As is well known, a magnet coil can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance of the magnet coil to negligible levels, such that when a power source is initially connected to the coil (for a period, for example, of 100 minutes) to introduce a current flow through the coil, the current will continue to flow through the coil due to the negligible coil resistance even after power is removed, thereby maintaining a strong magnetic field. Superconducting magnets find wide application, for example, in the field of MRI.
In a typical MRI superconducting magnet, the pressure vessel, which is contained within a vacuum vessel, is surrounded by a plurality of intermediate concentric heat shields. Each successive heat shield is at a slightly higher temperature than the cryogen in order to thermally isolate the pressure vessel from ambient temperatures on the outside of the vacuum vessel, which may be in the order to some 300.degree. K. higher than the cryogen temperature. The thermal shields are maintained at their cold temperatures by a cryogenic refrigerator which typically is contained within a stainless steel cold head sleeve cylinder with thermal connections between the cold head and the thermal shields. The thermal connection between the cold head and the thermal shields must be thermally conductive and efficient, and operate in the presence of vibration generated by the movement of the piston and gas within the cryogenic refrigerator or cryocooler and the resultant movement of the cold head relative to the heat shields and relative to the superconducting magnet.
Considerable research has been directed at eliminating the need to continuously replenish the boiling helium with the associated problems of transporting and storing liquid helium. This has led to the further use of cryocoolers in recondensing apparatus used to recondense the helium gas back to liquid helium for reuse in providing the superconducting temperatures.
Cryocoolers have been found to provide two major problems, acoustic noise and vibration. The noise has proven to be nuisance and distraction to MRI operators and patients because of its disturbing frequency and the vibration has resulted in deterioration in the MRI image quality. An additional problem is minimizing noise and vibration without deterioration in the cryocooler performance.
U.S. Pat. No. 5,317,879 entitled, "Flexible Thermal Connection System Between A Cryogenic Refrigerator and An MRI Superconducting Magnet", assigned to the same assignee as the present invention, has helped particularly in reducing vibration transmitted to the thermal shields, but has not proven adequate in reducing cryocooler noise and vibration transmittal to the remainder of the superconducting magnet, particularly the vacuum vessel to which it is secured, and adequately reducing the acoustic noise generated by cryocooler operation.